The present invention relates to plastic industry machinery and more particularly to a loader for loading the hopper of a thermoplastic resin processing machine. Such processing machines include blow molding machines, injection molding machines, film extrusion machines, pipe extrusion machines and screw extrusion machines.
At the present time extrusion is one of the major methods of producing shaped articles from thermoplastic resin. In a widely used extrusion process thermoplastic resin, in the form of pellets, is delivered to a hopper at the top of a screw extrusion machine. For example, the pellets may be delivered to the extrusion factory in large bags and the bags dumped, by machine or by hand, into the hopper. The hopper has an orifice leading to an elongated chamber in which a screw compresses, and by friction heats, the resin pellets until the thermoplastic material becomes a hot formless mass. The thermoplastic assumes the shape of the opening at the extruder head and is cooled to retain that shape.
It is requently desirable to mix two types of plastic pellets to produce the extruded article. For example, it may be less costly to use some percentage of reclaimed resin pellets and the remainder virgin (unreclaimed) resin pellets. The two "types" of resin pellets may, alternatively, be pellets of two different colors or different thermoplastic compositions. Generally at least half virgin pellets are used and, depending upon the type and grade of thermoplastic and the articles to be produced, anywhere from 50 to 5% of reclaimed pellets may be employed. A similar mixing problem is presented when, for uniformity of color or other reasons, it is desired to mix together thermoplastic pellets from different bags or batches.
One method used to obtain such mixtures is to simply dump the different materials into the extrusion hopper one after another. The materials are put into the hopper in sequence and they may be poorly mixed in the hopper, producing a non-uniform "hit-and-miss" mixture which results in non-uniform products. In another method the two materials are mixed in a batch, either by hand or machine, and the mixed batch transferred to the hopper of the extruder. That mixing process requires careful supervision to assure the correct proportions and is time consuming and costly.
A device has been proposed in U.S. Pat. No. 3,309,146 which is a ratio loader. It automatically transfers, by vacuum, pg,3 two different thermoplastic pellet materials to a ratio loader device installed over the extruder hopper. The ratio of one type of resin pellet to another may be pre-set by the operator. That ratio loader suffers from certain serious defects, namely: (1) as the ratio of one material to another is determined by the heights of the different piles, one pile will generally (except in a 50-50 ratio) be higher than the other. When the load is unloaded the higher pile will continue to flow out after the lower pile has ceased to flow, resulting in a non-uniform mixture of material into the hopper of the extruder; (2) the method of controlling the amount of material causes a cone or dome shape to form on the top surface of the material and this cone or dome shape remains the same whether the proportion of material is small or large, resulting in inaccuracies in obtaining the desired ratios between the two materials.